Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
A defect scan may be performed on a number of disks as part of a quality control procedure when manufacturing the disks. If an excessive number of defects are detected, a disk may be discarded as unusable, or the manufacturing process of the disks may be modified in an attempt to reduce the number of defects. However, protrusion defects (e.g., due to an asperity on the disk) are typically considered more significant than recess defects (e.g., a pit on the disk) because a protrusion defect may damage the head due to a collision. It is therefore important to discriminate between protrusion defects and recess defects to avoid unnecessary reduction in yield or unnecessary modifications to the disk manufacturing process.
A defect scan may also be performed after installing a disk into a disk drive as part of a manufacturing procedure for the disk drive. If a defect is detected during the defect scan, the area associated with the defect may be mapped out as unusable. If the defect is caused by a protrusion on the disk (asperity), the area mapped out as defective may include a margin which prevents any part of the head from colliding with the protrusion. If a large number of protrusion defects are detected, the overall capacity of the disk may be significantly reduced. It is therefore desirable to discriminate between true protrusion defects and recess defects which may generate a signature similar to protrusion defects.